"Ladies and gentlemen, welcome to your first day here at Time Patrol Academy. No doubt you have questions. Yes, Jenkins?"
"Sir, I understand why I'm holding a primer on temporal physics and why this six volume overview of history is sitting on my desk. These other two books, though, what gives? The Book of Revelations with Commentary? And the Necronomicon?"
"You need to understand, Jenkins, that St. John of Patmos and Abdul Al-Hazrad were among the first two scholars to observe temporal paradox in the field. As you'll soon discover in Temp Phys 101, too much paradox in any one time period triggers the apocalypse."
"Excuse me, sir?"
"You see, the apocalypse is a five dimensional discontinuity in the structure of the universe. Too much paradox can trigger the endtimes at any point along the spacetime continuum, erasing everything that happens after that point. St. John and Al-Hazrad did their best to describe the initial symptoms of the apocalypse, though they lacked the scientific training to understand much of what they saw. Still, things like the dead rising and blood dripping from the sky really do happen during the initial stages of most observed apocalypses. In fact zombie attacks are the primary hazard you'll have to endure while attempting to undo temporal paradoxes in the field."